Add ability to convert a number to a cardinal numeral

lib/strings/numeral.rb

@@ -6,6 +6,37 @@ module Strings
   module Numeral
     class Error < StandardError; end
 
+    CARDINALS = {
+      0 => "",
+      1 => "one",
+      2 => "two",
+      3 => "three",
+      4 => "four",
+      5 => "five",
+      6 => "six",
+      7 => "seven",
+      8 => "eight",
+      9 => "nine",
+      10 => "ten",
+      11 => "eleven",
+      12 => "twelve",
+      13 => "thirteen",
+      14 => "fourteen",
+      15 => "fifteen",
+      16 => "sixteen",
+      17 => "seventeen",
+      18 => "eighteen",
+      19 => "nineteen",
+      20 => "twenty",
+      30 => "thirty",
+      40 => "forty",
+      50 => "fifty",
+      60 => "sixty",
+      70 => "seventy",
+      80 => "eighty",
+      90 => "ninety",
+    }
+
     CARDINAL_TO_SHORT_ORDINAL = {
       0 => "th",
       1 => "st",
@@ -22,6 +53,49 @@ class Error < StandardError; end
       9 => "th"
     }.freeze
 
+    SCALES = [
+      "hundreds-tens-ones",
+      "thousand",
+      "million",
+      "billion",
+      "trillion",
+      "quadrillion",
+      "quintillion",
+      "sextillion",
+      "septillion",
+      "octillion",
+      "nonillion",
+      "decillion",
+      "undecillion",
+      "duodecillion",
+      "tredecillion",
+      "quattuordecillion",
+      "quindecillion",
+      "sexdecillion",
+      "septemdecillion",
+      "octodecillion",
+      "novemdecillion",
+      "vigintillion"
+    ]
+
+    # Convert a number to a cardinal numeral
+    #
+    # @example
+    #   cardinalize(1234)
+    #   # => one thousand, two hundred thirty four
+    #
+    # @param [Numeric] num
+    #
+    # @return [String]
+    #
+    # @api public
+    def cardinalize(num)
+      convert_numeral(num)
+    end
+    module_function :cardinalize
+    alias :cardinalise :cardinalize
+    module_function :cardinalise
+
     def ordinalize(num)
       num.to_s + short_ordinalize(num)
     end
@@ -36,5 +110,83 @@ def short_ordinalize(num)
         CARDINAL_TO_SHORT_ORDINAL[num_abs % 10]
     end
     module_function :short_ordinalize
+
+    # Convert a number into a numeral
+    #
+    # @param [Numeric] num
+    #   the number to convert to numeral
+    #
+    # @return [String]
+    #   the number as numeral
+    #
+    # @api private
+    def convert_numeral(num)
+      n = num.to_i
+      words = []
+
+      SCALES.each_with_index do |scale, i|
+        mod = n % 1000
+
+        word = []
+        word << convert_hundreds(mod)
+        word << scale unless i.zero?
+
+        words.insert(0, word.join(" "))
+
+        n /= 1000
+
+        break if n.zero?
+      end
+
+      if words.join.empty?
+        return "zero"
+      end
+
+      words.join(", ")
+    end
+    module_function :convert_numeral
+
+    # Convert 3 digit number to equivalent word
+    #
+    # @return [String]
+    #
+    # @api private
+    def convert_hundreds(num)
+      word = []
+      hundreds = (num % 1000) / 100
+      tens = num % 100
+
+      if !hundreds.zero?
+        word << convert_tens(hundreds)
+        word << "hundred"
+      end
+
+      if !tens.zero?
+        word << convert_tens(tens)
+      end
+
+      word.join(" ")
+    end
+    module_function :convert_hundreds
+
+    # Convert number in 0..99 range to equivalent word
+    #
+    # @return [String]
+    #
+    # @api private
+    def convert_tens(num)
+      word = []
+      tens = num % 100
+
+      if tens.to_s.size < 2 || tens <= 20
+        word << CARDINALS[tens]
+      else
+        word << CARDINALS[(tens / 10) * 10]
+        word << CARDINALS[tens % 10]
+      end
+
+      word.join(" ")
+    end
+    module_function :convert_tens
   end # Numeral
 end # Strings

spec/unit/cardinalize_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+RSpec.describe Strings::Numeral, "#cardinalize" do
+  {
+    0 => "zero",
+    1 => "one",
+    20 => "twenty",
+    21 => "twenty one",
+    100 => "one hundred",
+    101 => "one hundred one",
+    111 => "one hundred eleven",
+    113 => "one hundred thirteen",
+    123 => "one hundred twenty three",
+    1234 => "one thousand, two hundred thirty four",
+    12345 => "twelve thousand, three hundred forty five",
+    123456 => "one hundred twenty three thousand, four hundred fifty six",
+    1234567 => "one million, two hundred thirty four thousand, five hundred sixty seven"
+  }.each do |num, word|
+    it "cardinalizes #{num.inspect} to #{word.inspect}" do
+      expect(Strings::Numeral.cardinalize(num)).to eq(word)
+    end
+  end
+end